1. Field of the Invention
The present invention relates to a charge coupled device and particularly to a charge coupled device such as a linear image sensor for use in a facsimile, an area image sensor for use in a still camera or a video camera and the like.
2. Description of the Prior Art
Charge coupled devices (hereinafter referred to as CCD) are utilized as various kinds of image sensors, which are roughly classified into two types to be utilized for a video camera: image sensors of a frame transfer system in which a CCD portion contains a photoelectric conversion portion, and image sensors of an interline transfer system in which a photoelectric conversion portion outside a CCD portion is coupled to the CCD portion. However, since such a CCD image sensor requires as many as approximately 200,000 photoelectric conversion elements, the demand for CCD of the frame transfer system having a simpler structure is so largely increasing these days that the frame transfer system is applied in most cases.
With a view to simplifying the structure and the operation of a CCD image sensor of the above described frame transfer system, a system called "Virtual Phase Technology" is proposed by Texas Instruments Inc. in U.S.A. (by J. Hynecek, in IEEE, Vol. ED-28, No. 5, May 1981). FIG. 1(a) shows a structure of electrodes of a CCD image sensor of such system and FIG. 1(b) shows a potential form thereof. Referring to FIG. 1(a), an oxide film O is formed on a P type silicon substrate S. On the oxide film O, a plurality of electrodes M, M, . . . are disposed in parallel. In the surface regions of the semiconductor substrate S positioned between the respective electrodes M, high concentration regions S.sub.1, S.sub.1, . . . are formed by ion implantation of a P type impurity having the same conductive type as that of the substrate S. In right half portions of the surface regions of the semiconductor substrate S under the respective electrodes M and in right half portions of the regions of the semiconductor substrate S under the respective high concentration regions S.sub.1, well regions S.sub.2, S.sub.2, . . . shown by the marks + are provided by ion implantation of an N type impurity having the conductive type opposite to that of the substrate S. Beforehand, on the whole surface layer of the substrate S, an N type impurity layer assigned for channel regions has been formed. On this N type impurity layer, high concentration regions are selectively formed and these high concentration regions become the well regions S.sub.2, S.sub.2, . . .
As for the impurity concentrations of the respective portions, the impurity concentration of the substrate S is selected to be approximately 10.sup.15 /cm.sup.3, that of the N type impurity layer formed on the surface layer of the substrate S is selected to be approximately 10.sup.16 /cm.sup.3, that of the high concentration regions in this N type impurity layer, namely, the well regions S.sub.2 is selected to be approximately 2.times.10.sup.16 /cm.sup.3 and that of the P type high concentration regions S.sub.1 is selected to be approximately 10.sup.20 /cm.sup.3, for example.
In a CCD of such structure, a clock pulse .phi. of one phase is applied to the respective electrodes M, while the high concentration regions S.sub.1 in the surface regions of the semiconductor substrate S serve substantially as fixed potential electrodes, generating a potential form as shown in FIG. 1(b). More specifically, in FIG. 1(b), the solid lines represent potential at the time when the clock pulse .phi. is in the high level and the broken lines represent potential at the time when the clock pulse .phi. is in the low level. As can be seen from FIG. 1(b), a potential under each electrode M and a potential under each high concentration region S.sub.1 as a fixed potential electrode are changed dependently on each well region S.sub.2 in the right half portion thereunder so that a potential well deeper than the left half portion is formed. Accordingly, carriers captured in a potential well in each well region S.sub.2 (namely, photoelectrically converted electrons in this case) are transferred successively to the potential well of the adjacent well region S.sub.2 on the right so that these electrons are read out through an output amplifier (not shown).
Such a conventional CCD as described above has advantages such as simplicity of one phase of the driving pulse; however, since well regions S.sub.2 need be selectively formed in advance under the respective electrodes M and under the respective high concentration regions S.sub.1 for the purpose of determining the electric charge transfer direction, a drawback is involved that the manufacturing process thereof becomes complicated. For example, when electrodes M and high concentration regions S.sub.1 are formed after well regions S.sub.2 have been formed in the semiconductor substrate S by ion implantation, as shown in FIG. 2(a), the precision of mask alignment might be decreased, causing deviation in positioning. In such case, the potential form would be distorted as shown in FIG. 2(b) and the carriers could not be transferred.
On the other hand, with regard to a capacity for transferring carriers, namely, photoelectrically converted electrons, this capacity is determined by the smaller value out of the potential energy differences V and V' determined in advance by an ion implantation amount in each well region S.sub.2. Accordingly, much inconvenience is caused in practical application since the operation conditions of such a CCD, as well as the conditions for selecting a photoelectric conversions amount in case of such a CCD serving as an image sensor, cannot be determined freely.